Add a port_var_install_notes and modify the template in order to make possible
to also add a link to the INSTALL notes for ports that have port_var-s.
Addresses port-evbmips/50956, also discussed with sevan@.

Extend port.mdwn template in order to accomodate more port_var<n> and
address PR 50956 reported by sevan@.
o Add port_var[3-7] to accomadate more variants.
o Avoid hard-coded logic that previously needed that exactly two port_var(s)
was needed. Now also from 0 to 7 port_var-s can be defined independently.
o Fix the title for ``[...] pkgsrc binary packages'' in ``[...] pre-built
binary packages from pkgsrc'' anchors.